Optical data storage system with differential data detection and source noise subtraction for use with magneto-optic, write-once and other optical media

Abstract

An apparatus and method for providing differential detection and source noise subtraction in data signals read from magneto-optic (MO), write-once (WO) or other types of optical storage media using an incident radiation beam. A source monitor signal is generated by detecting radiation from an optical source providing the radiation beam. An error detector generates an error signal corresponding to a difference between the source monitor signal and a reference signal. A power level of the radiation beam is adjusted in response to the error signal. A data signal generator is adapted to receive a return beam resulting from application of the radiation beam to the medium and to generate a WO data signal from the return beam. A signal combiner combines the data signal and the source monitor signal to provide a compensated WO data signal. An MO data signal may be generated using a separate AGC circuit to adjust the gain of each differential detection channel in accordance with low-frequency amplitude differences between the detected signals and a reference signal. The same reference signal may be used in both the source noise subtraction and the MO data signal generation.

We claim: 
     
       1. In an optical system in which data is read from an optical medium using an incident radiation beam, an apparatus comprising: a source monitor detector to generate a source monitor signal by detecting radiation from an optical source providing the radiation beam;   an error detector coupled between the source monitor detector and the optical source, the error detector operative to generate an error signal corresponding to a difference between the source monitor signal and a reference signal, wherein a power level of the radiation beam is adjusted in response to the error signal;   a data signal generator adapted to receive a return beam resulting from application of the radiation beam to the medium and to generate a data signal from the return beam; and   a signal combiner coupled to the source monitor detector and the data signal generator and operative to combine the data signal and the source monitor signal to provide a compensated data signal.   
     
     
       2. The apparatus of claim 1 wherein the data signal generator further includes: first and second detectors for detecting first and second portions of the return beam, respectively;   first and second gain control circuits coupled to the first and second detectors, respectively, and receiving first and second detected signals therefrom, respectively, wherein each of the circuits is operative to alter the gain of a signal path of the corresponding detected signal in response to a control signal; and   a signal difference circuit coupled to the first and second gain control circuits and responsive to the first and second detected signals from the gain control circuits to produce a magneto-optic data signal corresponding to a difference between the detected signals.   
     
     
       3. The apparatus of claim 2 wherein each of the first and second gain control circuits further includes: a variable gain amplifier connected to the corresponding detector and having a control signal input for receiving the control signal;   a low pass filter connected to an output of the variable gain amplifier; and   an integrating amplifier connected to the low pass filter and having a feedback path with a capacitive element therein, the integrating amplifier having a first input for receiving the detected signal from the low pass filter, a second input for receiving the reference signal and an output corresponding to the control signal.   
     
     
       4. The apparatus of claim 3 wherein the variable gain amplifiers in the gain control circuits are matched amplifiers in a multiple amplifier device. 
     
     
       5. The apparatus of claim 3 wherein the low pass filters have a cut-off frequency of about 100 kHz or less, such that the variable gains of the gain control circuits are controlled in response to low-frequency amplitude differences between the reference signal and the first and second detected signals, respectively. 
     
     
       6. The apparatus of claim 2 wherein the data signal generator further includes a summing amplifier coupled to the first and second gain control circuits and responsive to the first and second detected signals from the gain control circuits to produce a write-once data signal corresponding to a sum of the first and second detected signals. 
     
     
       7. The apparatus of claim 1 wherein the reference signal is a predetermined reference voltage level. 
     
     
       8. The apparatus of claim 1 further including a delay element arranged in a signal path of at least one of the source monitor signal and the data signal to substantially match a phase of the source monitor signal and the data signal at an input of the signal combiner. 
     
     
       9. The apparatus of claim 1 wherein the error detector further includes: a preamplifier connected to the source monitor detector and adapted to receive the source monitor signal therefrom;   a low pass filter connected to the output of the preamplifier and adapted to provide an output including low-frequency components of the source monitor signal; and   an integrating amplifier having a first input coupled to the output of the low pass filter, a second input coupled to the reference signal, and an output coupled to the optical source.   
     
     
       10. The apparatus of claim 1 wherein the data signal generator further includes: a data detector adapted to receive the return beam and to generate a data signal therefrom; and   a gain control circuit adapted to receive the data signal and to adjust a signal level thereof in response to a control signal.   
     
     
       11. The apparatus of claim 1 further including an amplifier coupled between the source monitor detector and an input of the signal combiner, wherein the amplifier provides a gain of approximately two. 
     
     
       12. In an optical system in which data is read from an optical medium using an incident radiation beam, a method comprising the steps of: generating a source monitor signal using radiation from an optical source providing the radiation beam;   generating an error signal corresponding to a difference between the source monitor signal and a reference signal;   controlling a power level of the radiation beam in response to the error signal;   generating a data signal from a return beam resulting from application of the radiation beam to the medium; and   combining the data signal and the source monitor signal to provide a compensated data signal.   
     
     
       13. The method of claim 12 wherein first and second portions of the return beam are detected in first and second detectors, respectively, and the step of generating the data signal further includes the steps of: controlling the gain of a signal path of the first detected signal in response to a first control signal;   controlling the gain of a signal path of the second detected signal in response to a second control signal; and   generating a magneto-optic data signal corresponding to a difference between the detected signals.   
     
     
       14. The method of claim 13 wherein the steps of controlling the gain each further include the steps of: providing a variable gain amplifier in the corresponding signal path, the variable gain amplifier having a control signal input for receiving the corresponding control signal;   low pass filtering an output of the variable gain amplifier; and   generating the control signal by comparing the low pass filtered output of the variable gain amplifier to the reference signal.   
     
     
       15. The method of claim 12 wherein step of generating an error signal corresponding to a difference between the source monitor signal and a reference signal includes generating an error signal corresponding to a difference between the source monitor signal and a predetermined reference voltage level. 
     
     
       16. The method of claim 12 further including the step of substantially matching a phase of the data signal and the source monitor signal prior to the step of combining said signals to provide a compensated data signal. 
     
     
       17. The method of claim 12 wherein the step of generating an error signal further includes the steps of: amplifying the source monitor signal;   low pass filtering the amplified source monitor signal to provide an output including low-frequency components of the source monitor signal; and   integrating a difference between an amplitude of the low-frequency components and the reference signal.   
     
     
       18. The method of claim 12 wherein the step of generating a data signal further includes the step of summing first and second detected signals, corresponding to first and second portions of the return beam, to provide a write-once data signal. 
     
     
       19. The method of claim 18 further including providing additional gain in a signal path of the source monitor signal. 
     
     
       20. The method of claim 12 wherein the step of generating a data signal from the return beam further includes the steps of: detecting the return beam in a data detector to provide the data signal; and   adjusting a signal level of the data signal in response to a control signal.   
     
     
       21. An apparatus in which data is read from an optical storage medium using a radiation beam, the apparatus comprising: a source monitor detector to generate a source monitor signal by detecting radiation from an optical source providing the radiation beam;   an error detector coupled between the source monitor detector and the optical source, the error detector operative to generate an error signal corresponding to a difference between the source monitor signal and a reference signal, wherein a power level of the radiation beam is adjusted in response to the error signal;   first and second differential detectors for generating first and second detected signals, respectively, from first and second portions of a return beam resulting from application of the radiation beam to the medium;   a first gain control circuit adapted to receive the first detected signal and to adjust a signal level thereof in response to a first control signal;   a second gain control circuit adapted to receive the second detected signal and to adjust a signal level thereof in response to a second control signal;   a signal difference circuit coupled to the first and second gain control circuits and responsive to the first and second detected signals from the gain control circuits to produce a magneto-optic data signal corresponding to a difference between the detected signals;   a summing circuit coupled to the first and second gain control circuits and responsive to the first and second detected signals from the gain control circuits to produce a write-once data signal corresponding to a sum of the first and second detected signals; and   a signal combiner coupled to the source monitor detector and the summing circuit and operative to combine the write-once data signal and the source monitor signal to provide a compensated write-once data signal.   
     
     
       22. An apparatus in which data is read from an optical storage medium using a radiation beam, the apparatus comprising: a source monitor detector to generate a source monitor signal by detecting radiation from an optical source providing the radiation beam;   an error detector coupled between the source monitor detector and the optical source, the error detector operative to generate an error signal corresponding to a difference between the source monitor signal and a reference signal, wherein a power level of the radiation beam is adjusted in response to the error signal;   a data detector adapted to receive a return beam resulting from application of the radiation beam to the medium, and to generate a data signal from the return beam;   a gain control circuit adapted to receive the data signal and to adjust a signal level thereof in response to a control signal; and   a signal combiner coupled to the source monitor detector and the gain control circuit and operative to combine the data signal and the source monitor signal to provide a compensated data signal.